This invention relates to an automatic focusing device, and more particularly to an automatic focusing device for use in optical recording apparatus.
An optical disk apparatus attracts much attention, because it can record data with a high density. The apparatus is used in, for example, a document filing system which records much image information.
The optical disk apparatus uses an optical disk which has tracking prepits. The prepits are arranged to form circular tracks or a spiral track. Data are recorded on each track by directing an intense recording laser beam toward the rotating optical disk. The recorded data are read out by directing a less intense reading laser beam toward the rotating optical disk. During recording or reading the tracking of an optical head is controlled by detecting the tracking prepits. The detection of prepits is performed also by directing a laser beam toward the optical disk.
More specifically, data are recorded by intermittently radiating the recording laser beam on the optical disk from the optical head, while the tracking control is being conducted, thereby cutting data pits along the track and among the tracking pits. The data thus recorded are read out by radiating on the rotating disk the reading laser beam, while the tracking control is being carried out, thereby generating data pit signals. The data pit signals are processed to thereby reproduce the recorded data.
The width of the track, i.e. the train of prepits, is approximately 1 .mu.m. The laser beam must therefore be focused so that the beam spot on the track has a diameter of about 1 .mu.m. The optical disk warps, however, though very little. The optical disk may be placed eccentric with the shafts of a motor for rotating it. Strictly speaking, its thickness is not uniform. Further, the optical disk may vibrate while rotating. As a result, the distance between the track of the rotating optical disk and the objective lens of the optical head will inevitably change. Changes of the distance, even if small, result in an erroneous landing of the laser beam on the track. In other words, changes of the distance will make it impossible to focus the beam right on the track. In consequence, an incorrect data recording or an incorrect data reading will be unavoidable.
In order to avoid such an incorrect data recording or reading as mentioned above, the distance between the optical disk and the objective lens must be kept unchanged all the time during the data recording or reading. This is achieved by optically detecting an erroneous landing of the beam and moving the objective lens according to the data resulting from the optical detection. This technique is called "automatic focusing".
The conventional automatic focusing comprises steps of detecting changes of the shape of a laser beam spot on the light-receiving surface of an optical sensor and moving the ovjective lens of an optical head in accordance with the changes detected. The changes of the shape of the beam spot are proportional to changes of the distance between an optical disk and the objective lens. The conventional automatic focusing, however, is disadvantageous. The laser beam reflected from data pits or apertures made in a thin recording metal layer forms a beam spot having a diffraction (or interference) pattern on the light-receiving surface of the optical sensor. The pattern is detected by the optical sensor as changes of shape of the laser beam. The optical head is thus moved according to the outputs from the optical sensor. Consequently, a correct beam focusing will become impossible particularly when the laser beam moves across a track or tracks in order to achieve a random access data recording.